Stage 3 – previously divided into stages 3 and 4, is deep sleep, slow-wave sleep (SWS). Stage 3 was formerly the transition between stage 2 and stage 4 where delta waves, associated with "deep" sleep, began to occur, while delta waves dominated in stage 4. In 2007, these were combined into just stage 3 for all of deep sleep.
Dreaming is more common in this stage than in other stages of NREM sleep though not as common as in REM sleep. The content of SWS dreams tends to be disconnected, less vivid, and less memorable than those that occur during REM sleep. This is also the stage during which parasomnias most commonly occur.
Dreaming during NREM
Though dreaming most commonly occurs during the rapid eye movement sleep stage based on there being more dream recall and vividness, dreaming can also occur during NREM sleep.
Research has also shown that dreams during the NREM stage most commonly occur during the morning hours which is also the time period with the highest occurrence of REM sleep. This was found through a study involving subjects taking naps over specific intervals of time and being forcefully awakened, their sleep was separated into naps including only REM sleep and only NREM sleep using polysomnography. This implies that the polysomnographic occurrence of REM sleep is not required for dreaming. Rather, the actual mechanisms that create REM sleep cause changes to ones' sleep experience. Through these changes, by morning, a sub-cortical activation occurs during NREM that is comparable to the type that occurs during REM. It is this sub-cortical activation that results in dreaming during the NREM stage during the morning hours.
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